The present invention concerns computer networks and, in particular, a method for router reachability verification for hosts on a network.
In mobile networks, for example, a mobile device (i.e., the host) such as a portable computer with a wireless local area network (WLAN) connection may connect to a network (e.g., the Internet) through a stationary access point. When the mobile device is moved, however, it may move to a location that is out of the range of its existing access point but within range of a new access point. Therefore, to prevent loss of connection to the network, the mobile device is “handed-off” from the old access point to the new. This hand-off occurs both on level 2 (i.e. the data link layer) and level 3 (i.e. the network layer) of the open system interconnect (OSI) network model. During the hand-off process, the mobile device may not be able to send and receive data packets until both the level 2 and level 3 connections are resolved with the new access point and router. This handoff latency is undesirable and may even prevent certain applications (e.g. voice over IP, streaming media, and real-time applications) from running due to the relatively long latency and consequent interruption of data flow between the mobile device and the network.
At level 2, the mobile device probes the new access point to identify a channel that it may use to communicate with the access point. Typically, wireless channels correspond to predetermined frequency bands that are defined depending on the communications protocol being used (e.g., 802.11). Because a mobile device may try several channels before finding one on which it can communicate with the new access point, the latency due to establishing a level 2 connection alone may be as much as 400-500 ms.
Once a level 2 connection is established, the mobile device may then configure itself (i.e., at the network level, level 3) with parameters appropriate for a new access router. This is typically done by having the mobile device send a router solicitation message in its new environment. When the router receives the router solicitation message, it responds with a router advertisement message. This message is not sent immediately upon receipt of the router solicitation message but is sent with a random delay to prevent flooding that may occur when multiple routers communicate on the same channel. The router solicitation message and router advertisement message are described in Internet Request for Comments (RFC) 2461 entitled “Neighbor Discovery for IP Version 6 (IPv6).”
When a mobile device attaches itself to a new layer-2 link, it needs to detect whether its layer-3 link has changed.
In many situations, a mobile device that initiates a new level 2 connection may receive router advertisements from more than one router in response to its router solicitation. However, some of the router advertisements may be from routers replying to the solicitations of other mobile devices. The current mobile device is not able to confirm bi-directional reachability with the router when it can not confirm whether the router advertisement it received is in response to its solicitation or to the solicitation from some other mobile device. RFC 2461 describes a method whereby a router may specifically uni-cast a router advertisement in response to each individual router solicitation from a plurality of hosts (e.g., mobile devices). This method, however, may undesirably increase traffic on a dynamic network where hosts are rapidly joining and leaving and thereby sending a large number of router solicitations, each of which must be responded to individually by the router.
Additionally, a mobile device may have been previously connected to a router that is still accessible once a new level 2 connection is established. If the mobile device attempts to configure itself using the first router advertisement received in response to its router solicitation, it may be configuring itself using a router advertisement from a new router, thereby requiring the mobile device to unnecessarily initiate a new level 3 configuration. These unnecessary re-configurations may be referred to as erroneous movement detection, since the mobile device may have continued to use its existing level 3 configuration.
Furthermore, each router of a plurality of routers may have to respond to each one of a plurality of router solicitations, undesirably increasing the data traffic in the communication channel. This problem is most evident in “hot spots,” for example, airport terminals or other public venues where hand-off traffic is likely to be relatively significant.
All of the methods of layer 3 configuration in the prior art, as described above, may result in undesirable and unnecessary layer 3 re-configurations as well as an undesirable increase in network traffic. Those skilled in the art will recognize that such methods and undesirable traits are not limited to mobile networks and wireless LANs.